The capacitance of capacitors generally can reduce over time (e.g. due to self-healing, corrosion, or some other aging effects). This reduction may occur inhomogenously, such that after some time, some parts of the capacitor may have lost more capacitance than other parts. Similarly, if multiple capacitors or capacitor segments are connected either in parallel or in series, after some time, some of these capacitors or segments may have lost more capacitance than others.
Rolled film capacitors having internal series connections are known. In these capacitors, a metallization layer, which is provided on a surface of the rolled film, is divided in a lateral direction (i.e. orthogonal to the winding direction), such as to form multiple separate electrodes. Thereby, two or more series connected capacitors are formed in a single winding, such as with some of the electrodes floating, (i.e., being electrically unconnected to terminals). The floating electrodes subdivide the potential drop between two electrodes connected to opposite leads.
It is in some cases desired to divide the metallization layer also in the longitudinal direction (i.e. in the winding direction) into segments, which are separated by insulating regions. For example, U.S. Pat. No. 5,057,967 describes a rolled film capacitor with a metal electrode pattern including a number of rectangular island electrodes mutually separated by a plurality of lateral and longitudinal insulating gaps and regularly arrayed in both directions. Further capacitors are known from EP 0 225 822, DE 198 06 586, EP 0 640 996, GB 2276765, EP 0 838 078, U.S. Pat. No. 4,348,714, and U.S. Pat. No. 5,442,516. All of the disclosure of these documents are incorporated herein by reference in their entireties.
The segmentation of commercially available metallized films is can be made as follows: Oil or a similar substance is transferred onto the film surface in a regular pre-determined pattern. Then, metal is evaporated and deposited for forming the segments. Since the metal can only be deposited wherever there is no oil on the surface, the regions covered by the oil remain insulating and thus divide the segments. The oil can be applied using a printing roll having a fixed circumference. The printing roll rolls in the longitudinal direction of the film, which is to become the winding direction of the capacitor. Hence, all patterns are self-repeating in the winding direction with a period of the circumference of the printing roll.
In the above capacitor, the distribution of currents and voltages in the winding and, hence, the voltage levels may vary considerably among the segments if their capacitance becomes inhomogeneous. This can have consequences on the lifetime performance of the capacitor. For example, if a segment becomes damaged and loses a considerable amount of its capacitance, high overvoltages may occur in regions near the damaged segment. These overvoltages may accelerate the aging of these near regions and finally damage them as well. Even if the segments are connected by narrow metal bridges known as gates, the above consequences can still occur, at least after the fuse gates have melted.